Glucose transport system GLUTs

GLUT 1 Human erythrocyte Blood-brain barrier Blood-retinal barrier Blood-placental barrier Blood-testis barrier Expressed in cell types with barrier functions a high-affinity glucose transport system... 

For the absorption of carbohydrates, amino acids, and peptides, a variety of transport systems following facilitated diffusion and active mechanisms have been identified on a molecular and functional level. D-Glucose is mainly absorbed via the Na -dependent transporter SGLTl in the brush-border membrane of enterocytes [18-20]. It is transported across the basolateral membrane by facilitated diffusion via the hexose transporter GLUT-2. Besides SGLTl, the Na +-independent transport protein GLUT-5 is localized in the apical enterocyte membrane, recognizing fructose as a substrate [21]. 

In the liver (and pancreas) a high rate of glucose uptake occurs after a meal. But instead of using the insulin-responsive GLUT-4 system, hepatocytes have a relatively large number of GLUT-2 transporters that are always active in the plasma membrane. Thus glucose transport into the hepatocytes is always rapid, and the concentration in the cytoplasm closely mirrors that in the bloodstream (i.e., plasma and red blood cells). 

Most known saturable transporters at the BBB are facilitated diffusion systems (Davson and Segal, 1996c). For example, GLUT-1, the transporter for glucose, is a facilitated diffusion transporter. If the level of glucose is artificially raised above that of serum (or if radioactive glucose is introduced into the CNS, but not the serum), efflux of glucose can be shown. 

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